1. Field of the Invention
This invention relates to detection apparatus and more specifically to apparatus for detecting the presence of processed photographic film in apparatus such as a film processor or printer.
2. Description of the Prior Art
In apparatus such as photographic printers, it is desirable to detect the presence of film in the film entrance slot of the printer. In the case of a minilab where the film is inserted by the customer, the film may be inserted in varying orientations. Also, the film may be curled in varying degrees and directions.
The prior art teaches the use of infrared detectors for detecting the presence of film in a film path. Such detectors typically operate in a beam interruption or beam reflection mode. In the beam interruption mode, the failure of an infrared light beam to reach a sensor indicates the presence of the film. In a beam reflection mode, the sensor senses the amount of infrared light reflected by the film.
Infrared detectors have disadvantages particularly when used in the reflective mode. In the reflective mode, the infrared detector is sensitive to the angle of the film relative to the sensor and to the amount of film curl. If the film is inserted by the customer, the angle is variable and uncontrolled. Also, if the negative is of low optical density, the infrared beam will pass through the film without detachable reflection.
In the case of a beam interruption mode, the accuracy of the detector will also be affected by the optical density, a low density allowing some of the beam light to pass through the film to the sensor to erroneously indicate that a film is not present.